The present invention relates to a thermoplastic elastomer composition wherein a silicone gum and a stabilizer are dispersed in a polyester resin and the silicone gum is dynamically vulcanized in the resulting mixture.
Thermoplastic elastomers (TPEs) are polymeric materials which possess both plastic and rubbery properties. They have elastomeric mechanical properties but, unlike conventional thermoset rubbers, they can be re-processed at elevated temperatures. This re-processability is a major advantage of TPEs over chemically crosslinked rubbers since it allows recycling of fabricated parts and results in a considerable reduction of scrap.
In general, two main types of thermoplastic elastomers are known. Block copolymer thermoplastic elastomers contain xe2x80x9chardxe2x80x9d plastic segments which have a melting point or glass transition temperature above ambient as well as xe2x80x9csoftxe2x80x9d polymeric segments which have a glass transition or melt point considerably below room temperature. In these systems, the hard segments aggregate to form distinct microphases and act as physical crosslinks for the soft phase, thereby imparting a rubbery character at room temperature. At elevated temperatures, the hard segments melt or soften and allow the copolymer to flow and to be processed like an ordinary thermoplastic resin.
Alternatively, a thermoplastic elastomer referred to as a simple blend, or physical blend, can be obtained by uniformly mixing an elastomeric component with a thermoplastic resin. When the elastomeric component is also cross-linked during mixing, a thermoplastic elastomer known in the art as a thermoplastic vulcanizate (TPV) results. Since the crosslinked elastomeric phase of a TPV is insoluble and non-flowable at elevated temperature, TPVs generally exhibit improved oil and solvent resistance as well as reduced compression set relative to the simple blends.
Typically, a TPV is formed by a process known as dynamic vulcanization, wherein the elastomer and the thermoplastic matrix are mixed and the elastomer is cured with the aid of a crosslinking agent and/or catalyst during the mixing process. A number of such TPVs are known in the art, including some wherein the crosslinked elastomeric component can be a silicone polymer while the thermoplastic component is an organic, non-silicone polymer (i.e., a thermoplastic silicone vulcanizate or TPSiV). In such a material, the elastomeric component can be cured by various mechanisms including radical, condensation and hydrosilylation method, but each method has its limitations.
Arkles, in U.S. Pat. No. 4,500,688, discloses semi-interpenetrating networks (semi-IPNs) wherein a vinyl-containing silicone fluid having a viscosity of 500 to 100,000 cS is dispersed in a conventional thermoplastic resin. Arkles only illustrates these IPNs at relatively low levels of silicone. The vinyl-containing silicone is vulcanized in the thermoplastic during melt mixing according to a chain extension or crosslinking mechanism which employs a silicon hydride-containing silicone component. Typical thermoplastics mentioned include polyesters, polyurethanes, styrenics, polyacetals and polycarbonates. This disclosure is expanded by Arkles in U.S. Pat. No. 4,714,739 to include the use of hybrid silicones which contain unsaturated groups and are prepared by reacting a hydride-containing silicone with an organic polymer having unsaturated functionality. Although Arkles discloses a silicone fluid content ranging from 1 to 40 weight percent (1 to 60% in the case of the ""739 patent), there is no suggestion of any criticality as to these proportions or to the specific nature of the organic resin.
Crosby et al., in U.S. Pat. No. 4,695,602, teach composites wherein a silicone semi-IPN vulcanized via a hydrosilation reaction is dispersed in a fiber-reinforced thermoplastic resin having a high flexural modulus. The silicones employed are of the type taught by Arkles, cited supra, and the composites are said to exhibit improved shrinkage and warpage characteristics relative to systems which omit the IPN.
Ward et al., in U.S. Pat. No. 4,831,07 1, disclose a method for improving the melt integrity and strength of a high modulus thermoplastic resin to provide smooth-surfaced, high tolerance profiles when the modified resin is melt-drawn. As in the case of the disclosures to Arkles et al., cited supra, a silicone mixture is cured via a hydrosilation reaction after being dispersed in the resin to form a semi-IPN, after which the resulting composition is extruded and melt-drawn.
U.S. Pat. No. 6,013,715 to Gornowicz et al. teaches the preparation of TPSiV elastomers wherein a silicone gum (or filled silicone gum) is dispersed in either a polyolefin or a poly(butylene terephthalate) resin and the gum is subsequently dynamically vulcanized therein via a hydrosilation cure system. The resulting elastomers exhibit an ultimate elongation at break of at least 25% and have significantly improved mechanical properties over the corresponding simple blends of resin and silicone gum in which the gum is not cured (i.e., physical blends). This is, of course, of great commercial significance since the vulcanization procedure, and the cure agents required therefor, add to both the complexity as well as the expense of the preparation and vulcanization would be avoided in many applications if essentially identical mechanical properties could be obtained without its employ. However, this patent specifically teaches that poly(ethylene terephthalate) resin, as well as other thermoplastic resins, could not be modified according to the disclosed method.
Although the above publications disclose the preparation of compositions using various thermoplastic resins as the matrix and a dispersed phase consisting of a silicone oil or elastomer which is dynamically vulcanized therein, neither these references, nor any art known to applicants, teach the preparation of TPSiV elastomers based on polyester resins having superior tensile and elongation properties, as disclosed herein.
It has now been discovered that TPSiV elastomers of the type described in above cited U.S. Pat. No. 6,013,715 can be prepared from various polyester resins, including poly(ethylene terephthalate). As in the case of the teachings of U.S. Pat. No. 6,013,715, the elastomers disclosed herein generally also have good appearance and have a tensile strength and/or elongation at least 25% greater than that of the corresponding simple (physical) blend wherein the diorganopolysiloxane is not cured. However, it has been surprisingly found that such properties are significantly enhanced when a minor portion of a stabilizer is incorporated in the formulation, this resulting in a TPSiV having an elongation of at least 30%. Furthermore, unlike the teachings of Arkles, cited supra, and others, the silicone component which is dispersed in the thermoplastic resin, and dynamically cured therein, must include a high molecular weight gum, rather than a low viscosity silicone fluid, the latter resulting in compositions having poor uniformity. Surprisingly, polyesters having a softening point greater than about 300xc2x0 C. could not be modified according to the present invention to prepare TPSiVs having the required 30% elongation.
The present invention, therefore, relates to a method for preparing a thermoplastic elastomer, said method comprising:
(I) mixing
(A) a thermoplastic resin comprising more than 50 percent by volume of a polyester resin other than poly(butylene terephthalate), said thermoplastic resin having a softening point of 23xc2x0 C. to 300xc2x0 C.,
(B) a silicone elastomer comprising
(Bxe2x80x2) 100 parts by weight of a diorganopolysiloxane gum having a plasticity of at least 30 and having an average of at least 2 alkenyl groups in its molecule and, optionally,
(Bxe2x80x3) up to 200 parts by weight of a reinforcing filler, the weight ratio of said silicone elastomer (B) to said thermoplastic resin (A) being greater than 35:65 to 85:15,
(C) 0.01 to 5 parts by weight of a stabilizer for each 100 parts by weight of said thermoplastic resin and said silicone elastomer,
(D) an organohydrido silicon compound which contains an average of at least 2 silicon-bonded hydrogen groups in its molecule and
(E) a hydrosilation catalyst, components (D) and (E) being present in an amount sufficient to cure said diorganopolysiloxane (Bxe2x80x2); and
(II) dynamically curing said diorganopolysiloxane (Bxe2x80x2), wherein said thermoplastic elastomer has an elongation of at least 30% and wherein at least one property of the thermoplastic elastomer selected from tensile strength or elongation is at least 25% greater than the respective property for a corresponding simple blend wherein said diorganopolysiloxane is not cured.
The invention further relates to a thermoplastic elastomer which is prepared by the above method.
Component (A) of the present invention is (Axe2x80x2) a saturated thermoplastic polyester resin, other than poly(butylene terephthalate), or a blend of at least one such polyester resin with (Axe2x80x3) a non-polyester, saturated thermoplastic resin, wherein the polyester resin (Axe2x80x2) comprises more than 50 percent of the blend volume. For the purposes of the invention, the polyester resin (Axe2x80x2) and the optional thermoplastic resin (Axe2x80x3) have a softening point of at least about 23xc2x0 C. but no greater than about 300xc2x0 C. Herein, the xe2x80x9csoftening pointxe2x80x9d corresponds to the respective melting point of the thermoplastic resin if this resin (or blend) is at least partially crystalline and corresponds to the glass transition temperature when the thermoplastic resin is completely amorphous. When the softening point is below about 23xc2x0 C., the resin is not a thermoplastic. Rather, such a material would already have an elastomeric character and modification thereof according to the method of the present invention would not be productive. On the other hand, a polyester resin, or resin blend, having a softening point greater than about 300xc2x0 C. can not be formulated into thermoplastic elastomers by the instant method. Preferably, the softening point is between 50xc2x0 C. and 300xc2x0 C. and most preferably between 200xc2x0 C. and 300xc2x0 C. Further, as used herein, the term xe2x80x9csaturated thermoplasticxe2x80x9d indicates that the resin does not contain aliphatic unsaturation.
Specific examples of resins which can comprise the saturated thermoplastic polyester of component (Axe2x80x2) include homopolymers such as poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), poly(ethylene naphthalate) (PEN), poly(butylene naphthalate) (PBN), and poly(cyclohexylenedimethylene terephthalate) (PCT), inter alia. Alternatively, the polyester (Axe2x80x2) can be a random, graft or block copolymer having more than 50 mole percent of its repeat units derived from the condensation of an organic diacid and an organic diol. Such copolymers can comprise (a) recurring structural units, wherein each structural unit comprises a diol residue and a diacid residue and wherein from 50 to 100 mol percent of the structural units comprise a divalent alicyclic hydrocarbon group as the diol residue of the structural unit (e.g., PCT and glycol modified polycyclohexyl terephthalate (PCT-G)); (b) recurring structural units, wherein each structural unit comprises a diol residue and a diacid residue and from 0 to less than 50% of the structural units comprise a divalent alicyclic hydrocarbon radical as the diol residue of the structural unit (e.g., PET-G); and (c) recurring structural units, wherein each structural unit comprises a diol residue and a diacid residue and wherein from 10 to less than 90% of the structural units comprise a divalent alicyclic hydrocarbon radical as the 3diol residue of the structural unit and from 10 to 90% of the structural units comprise a 2,6-naphythalene dicarboxylate group as the diacid residue of the structural unit. When the polyester resin contains two or more blocks having independent thermal transition temperatures, the above mentioned softening point refers to the higher melt point or glass transition temperature.
Saturated thermoplastic polyester resins are well known in the art and further description thereof is considered unnecessary.
It is preferred that the polyester resin is dried prior to use, as generally recommended by the manufacturer. This is typically accomplished by passing a dry air or inert gas stream over as-received resin pellets or powder at elevated temperatures. The degree of drying consistent with optimal ultimate elastomer properties depends on the particular polyester and other components of the invention and is readily determined by a few simple experiments for the system under consideration.
Optional thermoplastic resin (Axe2x80x3) is any saturated resin other than a polyester having a softening point of 23xc2x0 C. to 300xc2x0 C. The nature of this component is not critical provided it does not contain functional groups which would prevent the dynamic vulcanization of the silicone gum (Bxe2x80x3). It may be illustrated by thermoplastic resins such as polycarbonates (PC), acrylonitrile-butadiene-styrene terpolymers (ABS), polyamides, polystyrene, poly(phenylene oxide) (PPO), polypropylene (PP), thermoplastic polyolefins (TPO), polyetherimide (PEI) and polyketones, inter alia.
Silicone elastomer (B) is a diorganopolysiloxane gum (Bxe2x80x2) or, optionally, a uniform blend of this gum with a reinforcing filler (Bxe2x80x3).
Diorganopolysiloxane (Bxe2x80x2) is a high consistency (gum) polymer or copolymer which contains at least 2 alkenyl groups having 2 to 20 carbon atoms in its molecule. The alkenyl group is specifically exemplified by vinyl, allyl, butenyl, pentenyl, hexenyl and decenyl. The position of the alkenyl functionality is not critical and it may be bonded at the molecular chain terminals, in non-terminal positions on the molecular chain or at both positions. It is preferred that the alkenyl group is vinyl or hexenyl and that this group is present at a level of 0.001 to 3 weight percent, preferably 0.01 to 1 weight percent, in the diorganopolysiloxane gum.
The remaining (i.e., non-alkenyl) silicon-bonded organic groups in component (Bxe2x80x2) are independently selected from hydrocarbon or halogenated hydrocarbon groups which contain no aliphatic unsaturation. These may be specifically exemplified by alkyl groups having 1 to 20 carbon atoms, such as methyl, ethyl, propyl, butyl, pentyl and hexyl; cycloalkyl groups, such as cyclohexyl and cycloheptyl; aryl groups having 6 to 12 carbon atoms, such as phenyl, tolyl and xylyl; aralkyl groups having 7 to 20 carbon atoms, such as benzyl and phenethyl; and halogenated alkyl groups having 1 to 20 carbon atoms, such as 3,3,3-trifluoropropyl and chloromethyl. It will be understood, or course, that these groups are selected such that the diorganopolysiloxane gum (Bxe2x80x2) has a glass temperature (or melt point) which is below room temperature and the gum is therefore elastomeric. Methyl preferably makes up at least 50, more preferably at least 90, mole percent of the non-alkenyl silicon-bonded organic groups in component (Bxe2x80x2).
Thus, diorganopolysiloxane (Bxe2x80x2) can be a homopolymer or a copolymer containing such organic groups. Examples include gums comprising dimethylsiloxy units and phenylmethylsiloxy units; dimethylsiloxy units and diphenylsiloxy units; and dimethylsiloxy units, diphenylsiloxy units and phenylmethylsiloxy units, among others. The molecular structure is also not critical and is exemplified by linear and partially branched straight-chain, linear structures being preferred.
Specific illustrations of organopolysiloxane (Bxe2x80x2) include:
trimethylsiloxy-endblocked dimethylsiloxane-methylhexenylsiloxane copolymers;
dimethylhexenylsiloxy-endblocked dimethylsiloxane-methylhexenylsiloxane copolymers;
trimethylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane copolymers;
trimethylsiloxy-endblocked methylphenylsiloxane-dimethylsiloxane-methylvinylsiloxane
copolymers; dimethylvinylsiloxy-endblocked dimethylpolysiloxanes;
dimethylvinylsiloxy-endblocked dimethylsiloxane-methylvinylsiloxane copolymers;
dimethylvinylsiloxy-endblocked methylphenylpolysiloxanes;
dimethylvinylsiloxy-endblocked methylphenylsiloxane-dimethylsiloxane-methylvinylsiloxane copolymers; and similar copolymers wherein at least one end group is dimethylhydroxysiloxy. Preferred systems for low temperature applications include methylphenylsiloxane-dimethylsiloxane-methylvinylsiloxane copolymers and diphenylsiloxane-dimethylsiloxane-methylvinylsiloxane copolymers, particularly wherein the molar content of the dimethylsiloxane units is about 93%.
Component (Bxe2x80x2) may also include combinations of two or more organopolysiloxanes. Most preferably, component (Bxe2x80x2) is a polydimethylsiloxane homopolymer which is terminated with a vinyl group at each end of its molecule or is such a homopolymer which also contains at least one vinyl group along its main chain.
For the purposes of the present invention, the molecular weight of the diorganopolysiloxane gum is sufficient to impart a Williams plasticity number of at least about 30 as determined by the American Society for Testing and Materials (ASTM) test method 926. The plasticity number, as used herein, is defined as the thickness in millimetersxc3x97100 of a cylindrical test specimen 2 cm3 in volume and approximately 10 mm in height after the specimen has been subjected to a compressive load of 49 Newtons for three minutes at 25xc2x0 C. When the plasticity of this component is less than about 30, as in the case of the low viscosity fluid siloxanes employed by Arkles, cited supra, the TPSiVs prepared by dynamic vulcanization according to the instant method exhibit poor uniformity such that at high silicone contents (e.g., 50 to 70 weight percent) there are regions of essentially only silicone and those of essentially only thermoplastic resin, and the compositions are weak and friable. These gums are considerably more viscous than the silicone fluids employed in the prior art. For example, silicones contemplated by Arkles, cited supra, have an upper viscosity limit of 100,000 cS (0.1 m2/s) and, although the plasticity of fluids of such low viscosity are not readily measured by the ASTM D 926 procedure, it was determined that this corresponds to a plasticity of approximately 24. Although there is no absolute upper limit on the plasticity of component (Bxe2x80x2), practical considerations of processability in conventional mixing equipment generally restrict this value. Preferably, the plasticity number should be about 100 to 200, most preferably about 120to 185.
Methods for preparing high consistency unsaturated group-containing polydiorganosiloxanes are well known and they do not require a detailed discussion in this specification. For example, a typical method for preparing an alkenyl-functional polymer comprises the base-catalyzed equilibration of cyclic and/or linear diorganopolysiloxanes in the presence of similar alkenyl-functional species.
Component (Bxe2x80x3) is a finely divided filler which is known to reinforce diorganopolysiloxane (Bxe2x80x2) and is preferably selected from finely divided, heat stable minerals such as fumed and precipitated forms of silica, silica aerogels and titanium dioxide having a specific surface area of at least about 50 m2/gram. The fumed form of silica is a preferred reinforcing filler based on its high surface area, which can be up to 450 m2/gram and a fumed silica having a surface area of 50 to 400 m2/g, most preferably 200 to 380 m2/g, is highly preferred. Preferably, the fumed silica filler is treated to render its surface hydrophobic, as typically practiced in the silicone rubber art. This can be accomplished by reacting the silica with a liquid organosilicon compound which contains silanol groups or hydrolyzable precursors of silanol groups. Compounds that can be used as filler treating agents, also referred to as anti-creeping agents or plasticizers in the silicone rubber art, include such ingredients as low molecular weight liquid hydroxy- or alkoxy-terminated polydiorganosiloxanes, hexaorganodisiloxanes, cyclodimethylsilazanes and hexaorganodisilazanes. It is preferred that the treating compound is an oligomeric hydroxy-terminated diorganopolysiloxane having an average degree of polymerization (DP) of 2 to about 100, more preferably about 2 to about 10, and it is used at a level of about 5 to 50 parts by weight for each 100 parts by weight of the silica filler. When component (Bxe2x80x2) is the preferred vinyl-functional or hexenyl-functional polydimethylsiloxane, this treating agent is preferably a hydroxy-terminated polydimethylsiloxane.
When reinforcing filler (Bxe2x80x3) is employed, it is added at a level of up to 200 parts by weight, preferably 5 to 150 and most preferably 20 to 100 parts by weight, for each 100 parts by weight of gum (Bxe2x80x2) to prepare silicone elastomer (B). Such a blend is commonly termed a xe2x80x9cbasexe2x80x9d by those skilled in the silicone art. Blending is typically carried out at room temperature using a two-roll mill, internal mixer or other suitable device. Alternatively, a reinforcing filler-containing silicone elastomer can be formed in-situ during mixing, but prior to dynamic vulcanization of the gum, as further described infra. In the latter case, the temperature of mixing is kept below the melting point of the polyester resin until the reinforcing filler is well dispersed in the diorganopolysiloxane gum.
Stabilizer (C) is at least one organic compound selected from hindered phenols; thioesters; hindered amines; 2,2xe2x80x2-(1,4-phenylene)bis(4H-3, 1-benzoxazin-4-one); or 3,5-di-tert-butyl-4-hydroxybenzoic acid, hexadecyl ester. We have surprisingly observed that other classes of stabilizers recommended for formulating polyester compositions, such as organophosphites, do not impart the significant improvement in mechanical properties obtained with the stabilizers of the invention. Moreover, as shown in the examples below, some of the organophosphites actually contribute to the premature crosslinking of the silicone gum (i.e., before any catalyst is introduced), thereby interfering with the intended dynamic vulcanization.
For the purposes of the present invention, a hindered phenol is an organic compound having at least one group of the formula 
in its molecule, wherein Q is a monovalent organic group having 1 to 24 carbon atoms selected from hydrocarbon groups, hydrocarbon groups which optionally contain heteroatoms selected from sulfur, nitrogen or oxygen or halogen-substituted versions of the aforementioned groups. Examples of Q include groups such as alkyl, aryl, alkylaryl, arylalkyl, cycloalkyl and halogen-substituted version thereof; alkoxy groups having 1 to 24 carbon atoms, such as methoxy or t-butoxy; and hydrocarbon groups having 2 to 24 carbon atoms which contain heteroatoms (e.g., xe2x80x94CH2xe2x80x94Sxe2x80x94Rxe2x80x3, xe2x80x94CH2xe2x80x94Oxe2x80x94Rxe2x80x3 or xe2x80x94CH2xe2x80x94C (O)ORxe2x80x3, wherein Rxe2x80x3 is a hydrocarbon group having 1 to 18 carbon atoms). Further, although not explicitly shown in formula (i), it is also contemplated that the benzene ring may additionally be substituted with one or more of the above described Q groups. The residue of the organic compound to which group (i) is chemically bonded is not critical as long as it does not contain moieties which would interfere with the dynamic vulcanization, described infra. For example, this residue may be a hydrocarbon, a substituted hydrocarbon or a hetero atom-containing hydrocarbon group of the appropriate valence. It is also contemplated that the group according to formula (i) can be attached to hydrogen to form an organophenol. Preferably, the hindered phenol compound has a number average molecular weight of less than about 3,000.
A preferred hindered phenol compound contains at least one group of the formula 
in its molecule wherein the benzene ring may be optionally further substituted with hydrocarbon groups having 1 to 24 carbon atoms. In formula (ii), R is an alkyl group having one to four carbon atoms and Rxe2x80x2 is a hydrocarbon group having 4 to 8 carbon atoms.
Preferably, one to four of the groups shown in structures (i) or (ii) are attached to an organic residue of appropriate valence such that the contemplated compound has a molecular weight (MW) of less than about 1,500. Most preferably, four such groups are present in component (C) and this compound has a molecular weight of less than about 1,200. This monovalent (or polyvalent) organic residue can contain one or more heteroatoms such as oxygen, nitrogen, phosphorous and sulfur. The Rxe2x80x2 groups in the above formula may be illustrated by t-butyl, n-pentyl, butenyl, hexenyl, cyclopentyl, cyclohexyl and phenyl. It is preferred that both R and Rxe2x80x2 are t-butyl. For the purposes of the present invention, a group according to formula (ii) can also be attached to hydrogen to form a diorganophenol.
Non-limiting specific examples of suitable hindered phenols include 1,1,3-Tris(2xe2x80x2-methyl-4xe2x80x2-hydroxy-5xe2x80x2-t-butylphenyl)butane, N,Nxe2x80x2-hexamethylene bis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionamide), 4,4xe2x80x2-thiobis(2-t-butyl-5-methylphenol), 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethyl benzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione, N,Nxe2x80x2-hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide), tetrakis(methylene(3,5-di-tert-butyl-4-hydroxy-hydrocinnamate))methane, 1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl) benzene, 4,4xe2x80x2-methylenebis (2,6-di-tertiary-butylphenol), 2,2xe2x80x2-thiobis(6-tert-butyl-4-methylphenol), 2,2xe2x80x2-thiobis(4-octylphenol), 4,4xe2x80x2-thiobis(6-tert-butyl-2-methylphenol), 4,4xe2x80x2-thiobis(3,6-di-sec-amylphenol), 2-(4,6-bis(2,4-dimethylphenyl)-1,3,5,-triazin-2-yl)-5-(octyloxy) phenol, 2,4-bisoctylmercapto-6-(3,5-di-tert-butyl-4-hydroxyanilino)-1,3,5-triazine, 2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenoxy)-1,2,3-triazine, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 2-octylmercapto-4,6-bis(3,5-di-tert-butyl-4-hydroxyanilino)-1,3,5-triazine, 2-octylmercapto-4,6-bis(3,5-di-tert-butyl-4-hydroxyphenoxy)-1,3,5-triazine, 2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenylethyl)-1,3,5-triazine, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxyphenylpropionyl)hexahydro-1,3,5-triazine, 1,3,5-tris(3,5-dicyclohexyl-4-hydroxybenzyl) isocyanurate, 1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanurate, 2,6-di-tert-butyl-4-methoxyphenol, 2,5-di-tert-butylhydroquinone, 2,5-di-tert-amylhydroquinone, 2,6-di-tert-butylhydroquinone, 2,5-di-tert-butyl-4-hydroxyanisole, 2,6-diphenyl-4-octadecyloxyphenol, 3,5-di-tert-butyl-4-hydroxyanisole, 3,5-di-tert-butyl-4-hydroxyphenyl stearate, bis(3,5-di-tert-butyl-4-hydroxyphenyl) adipate, esters of beta-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid with mono- or polyhydric alcohols (e.g., methanol, ethanol, n-octanol, trimethylhexanediol, isooctanol, octadecanol, 1,6-hexanediol, 1,9-nonanediol, ethylene glycol, 1,2-propanediol, neopentyl glycol, thiodiethylene glycol, diethylene glycol, triethylene glycol, pentaerythritol, trimethylolpropane, tris(hydroxyethyl) isocyanurate, N,Nxe2x80x2-bis(hydroxyethyl)oxalamide, 3-thiaundecanol, 3-thiapentadecanol, 4-hydroxymethyl-1-phospha-2,6,7-trioxabicyclo(2.2.2) octane and esters of beta-(5-tert-butyl-4-hydroxy-3-methylphenyl)propionic acid with mono- or polyhydric alcohols (as above).
Thioesters of the invention are compounds having at least one group of the formula
Gxe2x80x94Sxe2x80x94Gxe2x80x83xe2x80x83(iii)
wherein G is xe2x80x94CH2xe2x80x94CH2xe2x80x94C(O)ORxe2x80x2xe2x80x3 and Rxe2x80x2xe2x80x3 is a monovalent hydrocarbon group having 1 to 24 carbon atoms. Specific non-limiting examples of suitable thioesters include distearyl 3,3xe2x80x2-thiodipropionate, dilauryl-3,3xe2x80x2-thiodipropionate and di(tridecyl)3,3xe2x80x2-thiodipropionate.
The hindered amine of the present invention is a low molecular weight organic compound or a polymer which contains at least one divalent group of the formula 
wherein Me hereinafter denotes a methyl group. The backbone of this component is not critical as long as it does not contain functionality which would interfere with the dynamic vulcanization of the silicone gum and it may be illustrated by low-molecular and polymeric polyalkylpiperidines, as disclosed in U.S. Pat. No. 4,692,486, hereby incorporated by reference. Preferably, the above group has the structure 
wherein Z is selected from hydrogen or an alkyl group having 1 to 24 carbon atoms, preferably hydrogen.
Specific non-limiting examples of suitable hindered amines include: 1,6-hexanediamine, N,Nxe2x80x2-bis(2,2,6,6-pentamethyl-4-piperidinyl)-, polymers with morpholine-2,4,6-trichloro-1,3,5-triazine; 1,6-hexanediamine, N,Nxe2x80x2-bis(2,2,6,6-tetramethyl-4-piperidinyl)-, polymers with 2,4,-Dichloro-6-(4-morpholinyl)-1,3,5-triazine; bis(1,2,2,6,6-pentamethyl-4-piperidinyl) sebacate; bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate; dimethyl succinate polymer with 4-hydroxy-2,2,6,6-tetramethyl-1-piperidine ethanol; and polymethyl (propyl-3-oxy-(2xe2x80x2,2xe2x80x2,6xe2x80x2,6xe2x80x2-tetramethyl-4xe2x80x2-piperidinyl) siloxane.
Preferred stabilizers of the invention are tetrakis(methylene(3,5-di-tert-butyl-4-hydroxy-hydrocinnamate))methane, N,Nxe2x80x2-hexamethylenebis(3,5-di-tert-butyl-4-hydroxyhydrocinnamamide) and 1,1,3-tris(2xe2x80x2-methyl-4xe2x80x2-hydroxy-5xe2x80x2-t-butylphenyl)butane.
From about 0.01 to about 5 parts by weight of stabilizer (C) are employed for each 100 parts by weight of resin (A) plus silicone elastomer (B). It has been found that the mechanical properties of the final thermoplastic elastomer are significantly diminished when less than about 0.01 part of component (C) is used. This is particularly true when the polyester has a relatively high melting point, such as is the case for PEN and PCT resins. On the other hand, when the content of component (C) is greater than about 5 weight parts for each 100 parts by weight of (A) plus (B), little additional benefit is imparted. We have found that, in general, more stabilizer is required as the softening point of the resin approaches the 300xc2x0 C. limit. With this guidance, the skilled artisan will readily determine the appropriate amount of stabilizer for any given system by routine experimentation. Preferably, 0.1 to 2 parts by weight, more preferably 0.1 to 1 part by weight, of (C) are added for each 100 parts by weight of (A) plus (B).
The organohydrido silicon compound (D) is a crosslinker (cure agent) for diorganopolysiloxane (Bxe2x80x2) of present composition and is an organopolysiloxane which contains at least 2 silicon-bonded hydrogen atoms in each molecule, but having at least about 0.1 weight percent hydrogen, preferably 0.2 to 2 and most preferably 0.5 to 1.7, percent hydrogen bonded to silicon. Those skilled in the art will, of course, appreciate that either component (Bxe2x80x2) or component (D), or both, must have a functionality greater than 2 if diorganopolysiloxane (Bxe2x80x2) is to be cured (i.e., the sum of these functionalities must be greater than 4 on average). The position of the silicon-bonded hydrogen in component (D) is not critical, and it may be bonded at the molecular chain terminals, in non-terminal positions along the molecular chain or at both positions. The silicon-bonded organic groups of component (D) are independently selected from any of the hydrocarbon or halogenated hydrocarbon groups described above in connection with diorganopolysiloxane (Bxe2x80x2), including preferred embodiments thereof. The molecular structure of component (D) is also not critical and is exemplified by straight-chain, partially branched straight-chain, branched, cyclic and network structures, linear polymers or copolymers being preferred.
Component (D) is exemplified by the following:
low molecular siloxanes, such as PhSi(OSiMe2H)3;
trimethylsiloxy-endblocked methylhydridopolysiloxanes;
trimethylsiloxy-endblocked dimethylsiloxane-methylhydridosiloxane copolymers;
dimethylhydridosiloxy-endblocked dimethylpolysiloxanes;
dimethylhydrogensiloxy-endblocked methylhydrogenpolysiloxanes;
dimethylhydridosiloxy-endblocked dimethylsiloxane-methylhydridosiloxane copolymers;
cyclic methylhydrogenpolysiloxanes;
cyclic dimethylsiloxane-methylhydridosiloxane copolymers;
tetrakis(dimethylhydrogensiloxy)silane;
silicone resins composed of (CH3)2HSiO1/2, (CH3)3SiO1/2, and SiO4/2 units; and
silicone resins composed of (CH3)2HSiO1/2, (CH3)3SiO1/2, CH3Si O3/2, PhSiO3/2 and SiO4/2 units, wherein Ph hereinafter denotes phenyl group.
Particularly preferred organohydrido silicon compounds are polymers or copolymers with Rxe2x80x3xe2x80x3 HSiO units ended with either Rxe2x80x3xe2x80x33SiO1/2 or HRxe2x80x3xe2x80x32SiO1/2, wherein Rxe2x80x3xe2x80x3 is independently selected from alkyl groups having 1 to 20 carbon atoms, phenyl or trifluoropropyl, preferably methyl. It is also preferred that the viscosity of component (D) is about 0.5 to 1,000 mPa- at 25xc2x0 C., preferably 2 to 500 mPa-s. Further, this component preferably has 0.5 to 1.7 weight percent hydrogen bonded to silicon. It is highly preferred that component (D) is selected from a polymer consisting essentially of methylhydridosiloxane units or a copolymer consisting essentially of dimethylsiloxane units and methylhydridosiloxane units, having 0.5 to 1.7 percent hydrogen bonded to silicon and having a viscosity of 2 to 500 mPaxe2x88x92s at 25xc2x0 C. Such a highly preferred system has terminal groups selected from trimethylsiloxy or dimethylhdridosiloxy groups. These SiH-functional materials are well known in the art and many of them are commercially available.
Component (D) may also be a combination of two or more of the above described systems and is used at a level such that the molar ratio of SiH therein to Si-alkenyl in component (Bxe2x80x2) is greater than 1 and preferably below about 50, more preferably 3 to 30, most preferably 4 to 20.
Hydrosilation catalyst (E) accelerates the cure of diorganopolysiloxane (Bxe2x80x2) in the present composition. This hydrosilation catalyst is exemplified by platinum catalysts, such as platinum black, platinum supported on silica, platinum supported on carbon, chloroplatinic acid, alcohol solutions of chloroplatinic acid, platinum/olefin complexes, platinum/alkenylsiloxane complexes, platinum/beta-diketone complexes, platinum/phosphine complexes and the like; rhodium catalysts, such as rhodium chloride and rhodium chloride/di(n-butyl)sulfide complex and the like; and palladium catalysts, such as palladium on carbon, palladium chloride and the like. Component (E) is preferably a platinum-based catalyst such as chloroplatinic acid; platinum dichloride; platinum tetrachloride; a platinum complex catalyst produced by reacting chloroplatinic acid and divinyltetramethyldisiloxane which is diluted with dimethylvinylsiloxy endblocked polydimethylsiloxane, prepared according to U.S. Pat. No. 3,419,593 to Willing; and a neutralized complex of platinous chloride and divinyltetramethyldisiloxane, prepared according to U.S. Pat. No. 5,175,325 to Brown et al. Most preferably, catalyst (E) is a neutralized complex of platinous chloride and divinyltetramethyldisiloxane.
Component (E) is added to the present composition in a catalytic quantity sufficient to promote the reaction of components (Bxe2x80x2) and (D) and thereby cure the diorganopolysiloxane to form an elastomer. The catalyst is typically added so as to provide about 0.1 to 500 parts per million (ppm) of metal atoms based on the total weight of the thermoplastic elastomer composition, preferably 0.25 to 100 ppm.
In addition to the above-mentioned components (A) through (E), a minor amount (i.e., less than about 40 weight percent of the total composition, preferably less than 20 weight percent) of an optional additive (F) can be incorporated in the compositions of the present invention. This optional additive can be illustrated by, but are not limited to, fillers, such as glass fibers and carbon fibers, quartz, talc, calcium carbonate, diatomaceous earth, iron oxide, carbon black and finely divided metals; lubricants; plasticizers; pigments; dyes; anti-static agents; blowing agents; heat stabilizers, such as hydrated cerric oxide; antioxidants; and fire retardant (FR) additives, such as halogenated hydrocarbons, alumina trihydrate, magnesium hydroxide and organophosphorous compounds. A preferred FR additive is calcium silicate particulate, preferably wollastonite having an average particle size of 2 to 30 xcexcm. The FR additive can be incorporated in the silicone gum (Bxe2x80x2) or in resin (A), or in both.
The above additives are typically added to the final thermoplastic composition after dynamic cure, but they may also be added at any point in the preparation provided they do not adversely affect dynamic vulcanization. Of course, the above additional ingredients are only used at levels which do not significantly detract from the desired properties of the final composition.
According to the method of the present invention, the thermoplastic elastomer is prepared by thoroughly mixing silicone elastomer (B) and stabilizer (C) with resin (A) and then dynamically vulcanizing the diorganopolysiloxane using organohydrido silicon compound (D) and catalyst (E). For the purposes of the present invention, the weight ratio of silicone elastomer (B) to resin (A) is greater than 35:65. It has been found that when this ratio is 35:65 or less, the resulting vulcanizate has a modulus more resembling that of thermoplastic resin (A) than that of a thermoplastic elastomer. On the other hand, the above mentioned ratio should be no more than 85:15 since the compositions tend to be weak and resemble cured silicone elastomers above this value. Notwithstanding this upper limit, the maximum weight ratio of (B) to (A) for any given combination of components is also limited by processability considerations since too high a silicone elastomer content results in at least a partially crosslinked continuous phase which is no longer thermoplastic. For the purposes of the present invention, this practical limit is readily determined by routine experimentation and represents the highest level of component (B) which allows the TPSiV to be compression molded. It is, however, preferred that the final thermoplastic elastomer can also be readily processed in other conventional plastic operations, such as injection molding and extrusion and, in this case, the weight ratio of components (B) to (A) should be no more than about 75:25. Such a preferred thermoplastic elastomer which is subsequently re-processed generally has a tensile strength and elongation similar to the corresponding values for the original TPSiV (i.e., the thermoplastic elastomer is little changed by this re-processing). Although the amount of silicone elastomer consistent with the above mentioned requirements depends upon the particular polyester resin and other components selected, it is preferred that the weight ratio of components (B) to (A) is 40:60 to 75:25, more preferably 40:60 to 70:30.
Mixing is carried out in any device which is capable of uniformly dispersing the components in the polyester resin or resin blend, such as an internal mixer or an extruder, the latter being preferred for commercial preparations, wherein the temperature is preferably kept as low as practical consistent with good mixing so as not to degrade the resin. Depending upon the particular system, order of mixing is generally not critical and, for example, components (A), (C) and (D) can be added to (B) at a temperature above the softening point of (A), catalyst (E) then being introduced to initiate dynamic vulcanization. However, components (B) through (D) should be well dispersed in resin (A) before dynamic vulcanization begins. As previously mentioned, it is also contemplated that a reinforcing filler-containing silicone elastomer can be formed in-situ. For example, the optional reinforcing filler may be added to a mixer already containing resin (A) and diorganopolysiloxane gum (Bxe2x80x2) at a temperature below the softening point of the resin to thoroughly disperse the filler in the gum. The temperature is then raised to melt the resin, the other ingredients are added and mixing/dynamic vulcanization are carried out. Optimum temperatures, mixing times and other conditions of the mixing operation depend upon the particular resin and other components under consideration and these may be determined by routine experimentation by those skilled in the art. It is, however, preferred to carry out the mixing and dynamic vulcanization under a dry, inert atmosphere (i.e., one that does not adversely react with the components or otherwise hider hydrosilation cure), such as dry nitrogen, helium or argon.
A preferred procedure according to the instant method comprises forming a pre-mix by blending dried polyester resin (A), silicone elastomer (B), stabilizer (C) and, optionally, organohydrido silicon compound (D) below the softening point of the resin (e.g., at ambient conditions). This pre-mix is then melted in a bowl mixer or internal mixer, preferably using a dry inert gas purge, at a controlled temperature which is just above the softening of the resin to about 35xc2x0 C. above this value and catalyst (E) is mixed therewith. Mixing is continued until the melt viscosity (mixing torque) reaches a steady state value, thereby indicating that dynamic vulcanization of the diorganopolysiloxane of component (B) is complete. Such a xe2x80x9ccold-blendxe2x80x9d procedure is particularly preferred when the melt point of the polyester resin is above about 280xc2x0 C., as in the case of, e.g., PCT resin.
As noted above, in order to be within the scope of the present invention, the tensile strength or elongation, or both, of the TPSiVs must be at least 25% greater than that of a corresponding simple blend. A further requirement of the invention is that the TPSiV has at least 30% elongation, as determined by the test described infra. In this context, the term xe2x80x9csimple blendxe2x80x9d or xe2x80x9cphysical blendxe2x80x9d denotes a composition wherein the weight proportions of resin (A), elastomer (B) and stabilizer (C) are identical to the proportions in the TPSiV, but no cure agents are employed (i.e., either component (D) or (E), or both, are omitted and the gum is therefore not cured). In order to determine if a particular composition meets the above criterion, the tensile strength of the TPSiV is measured on dumbbells having a length of 25.4 mm and a width of 3.2 mm and a typical thickness of 1 to 2 mm, according to ASTM method D 412, at an extension rate of 50 mm/min. Five samples are evaluated and the results averaged after removing obvious low readings due to sample inhomogeneity (e.g., such as voids, contamination or inclusions). These values are then compared to the corresponding average tensile and elongation values of a sample prepared from the simple blend composition. When at least a 25% improvement in tensile and/or elongation over the simple blend is not realized there is no benefit derived from the dynamic vulcanization and such compositions are not within the scope of the present invention.
The thermoplastic elastomer prepared by the above-described method can then be processed by conventional techniques, such as extrusion, vacuum forming, injection molding, blow molding or compression molding. Moreover, these compositions can be re-processed (recycled) with little or no degradation of mechanical properties.
The novel thermoplastic elastomers of the present invention can be used for fabricating parts and components for automotive, electronics, electrical, communications, appliance and medical applications, inter alia. For example, they may be used to produce wire and cable insulation; automotive and appliance components, such as belts, hoses, boots, bellows, gaskets, fuel line components and air ducts; architectural seals; bottle closures; furniture components; soft-feel grips for hand held devices (e.g. handles for tools); medical devices; sporting goods and general rubber parts.